Pixmap vs Bitmap

[Pedro Medeiros <pjira solar com br>]

GtkPixmap is basically a structure with a GdkPixmap and a
GdkBitmap, right? Then I create a GtkPixmap with:

 gdkpixmap = gdk_pixmap_create_from_xpm(window, &gdkbitmap,
                                        NULL, "file.xpm");
 pixmap = gtk_pixmap_new(gdkpixmap, gdkbitmap);

gdkbitmap and gdkpixmap seems to be properly initialized.
So why can't I use, for instance, something like this?

 gdk_window_get_size (pixmap->pixmap, &width, &height);
 gdk_draw_pixmap (window, gc, pixmap->mask, 0, 0,
                  0, 0, width, height);

The program exits and return the error:

Gdk-ERROR **: BadMatch (invalid parameter attributes)
  serial 315 error_code 8 request_code 72 minor_code 0

I can't display a bitmap although it's a valid pixmap.
Why is that?

--
Pedro Medeiros.